This invention relates to low voltage varistor arrays and methods for making the same.
A metal oxide varistor is a nonlinear electrical device--that is, it exhibits a nonlinear voltage-current behavior. Below a certain voltage, commonly called the breakdown voltage (V.sub.b) or the threshold voltage, the varistor is highly resistive, in the megohm range. When the breakdown voltage is exceeded, the resistance of the varistor decreases dramatically, and the varistor conducts large amounts of current. The voltage-current relationship of a varistor is described by the equation ##EQU1## where I is the current flowing through the varistor; V is the voltage across the varistor; C is a constant which is a function of the dimensions, composition, and method of fabrication of the varistor; and .alpha. is a constant which is a measure of the nonlinearity of the varistor. A large .alpha., signifying a large degree of nonlinearity, is desirable. High quality varistors typically have an .alpha. between 20 and 50.
Varistors generally are polycrystalline sintered ceramics of zinc oxide, containing additionally minor amounts of oxides of bismuth, cobalt, manganese, antimony, tin and/or other metals. Varistors can also be made of other materials such as strontium titanate or silicon carbide. The varistor precursor material is formed into a desired shape (the green body), optionally with the aid of binders, and sintered at an elevated temperature such as 1000.degree.-1400.degree. C. to develop the characteristic multigrain structure responsible for the varistor properties. The preparation of zinc oxide varistors has been extensively described, for example in Thompson et al., WO 88/02921 (1988) and Matsuoka et al., U.S. Pat. No. 3,496,512 (1970)
Varistors find a number of applications. Their use as surge arrestors is well known. Varistors have also been proposed as the switching elements in liquid crystal displays. Castleberry, U.S. Pat. No. 4,233,603 (1980), and Hareng et al., U.S. Pat. No. 4,535,327 (1985) disclose the use of varistors in multiplexed liquid crystal displays. Yoshimoto et al., EP 337,711 (1989), disclose varistors as switching elements in multiplexed encapsulated liquid crystal displays.
For liquid crystal display applications, varistors with lower breakdown voltages, on the order of about 60 volts, are advantageous. It is theorized that the breakdown voltage of a varistor is related to its thickness and the size of the crystalline grains. Each boundary between adjacent grains additively contributes a localized breakdown voltage to the overall breakdown voltage. For example, a varistor 1.5 mm thick and having an average grain size of 30 microns has an average of 50 grain boundaries across its thickness. Each grain boundary contributes about 3 volts to the overall breakdown voltage, so that such a varistor would have a breakdown voltage of about 150 volts.
One way to lower the breakdown voltage is to reduce the thickness of the varistor. However, the resulting varistor lacks mechanical strength, fracturing readily. In a multiplexed liquid crystal display application, each pixel is controlled by a varistor. A display having M rows and N columns of pixels would require M.times.N varistors. The failure of a few varistors among this large number can unacceptably degrade picture quality, so that rugged and reliable varistors are necessary.
An alternative way to reduce the breakdown voltage is to increase the grain size, thereby reducing the number of grain boundaries for a given thickness. Towards this end, the addition of titanium dioxide grain growth accelerators has been proposed. However, varistors so made are susceptible to current channeling through the varistor, due to local exaggerated grain growth. The result is nonuniform current conduction across the varistor's surface.
Levinson, in U.S. Pat. No. 4,364,021 (1982), discloses a varistor having a recessed region on one of its planar surfaces, to provide a region of reduced thickness and consequently lower breakdown voltage, while retaining the structural strength of a thicker varistor. However, Levinson relates to the preparation of individual varistors. For a multiplexed liquid crystal display, where a large array of varistors is required, one would then have to mount these individual varistors onto a supporting base, an inefficient step.
Kujawa et al., U.S. Pat. No. 3,195,091 (1965), discloses a silicon carbide non-linear resistor having plural leasds attached to recesses on one surface thereof and a single lead attached to the other surface thereof.
It is desirable to make a large array of varistors, where the varistors have a low breakdown voltage and are rugged and suitable for use with multiplexed liquid crystal displays. It is further desirable to make arrays of varistors which are integral, that is, wherein the varistor elements do not require a separate supporting base of a different material. These and other objectives are achieved by the present invention. In the method of the present invention the precursor material can be handled as a green body, as opposed to a less convenient precursor powder.